In the connection with the manufacture of paper it is very important to be able to control the porosity of the paper. For example, a paper with low porosity is required in order to obtain an acceptable result in, e.g., ink-jet and rotogravure printing. If the paper is too porous it will function like blotting paper during printing and the resulting print may appear blurred, the contrast between printed and unprinted areas or between differently coloured areas not being rendered sharply. Similarly, on a paper which is of non-uniform porosity it can be seen that the intensity of colouration varies (“mottling”), which is of course undesirable since the coloured surface appears variegated or mottled. On the other hand, the porosity of the paper can also be too low, since a very dense paper will have difficulty in absorbing printing ink, which among other things may result in smudging (“set off”) between printed sheets. This phenomenon can influence the printing results, the printing speed and the printing process employed in a negative manner.
The paper industry presently uses several different ways of regulating the porosity of paper. Use is made among other things of the fact that certain minerals in the form of flakes, e.g. talc and kaolin, will, as result of their form, be able to reduce the porosity since the individual particles will become deposited like the scales on a fish and thereby seal the surface. Fine silicates can be used in connection with pigmentation to reduce the porosity of the paper. When they come into or onto the paper, these fine particles will close the pores which contribute to the porosity of the paper.
In order to regulate the properties of the paper, a combination of one or more fillers and a variety of other additives is often used. Among the group of additives are alkylketene dimers (AKD), alkenylsuccinic acid anhydride (ASA), starch and retention agents. Retention agents are added to facilitate the manufacture of the paper, whilst AKD, ASA and starch are added to ensure the quality of the paper (strength, printing properties, etc.).
Regardless of which of the presently known methods is used, they all have drawbacks. Kaolin and talc in the form of flakes will negatively influence the brightness of the paper compared to the whiter fillers, such as ground marble or PCC (precipitated calcium carbonate).
The fine silicate products used for pigmentation have relatively good technical properties. However, the silicate products have the disadvantage of being much more expensive than the fillers normally used in paper manufacture. The same applies to other additives normally used in connection with paper manufacture. These are often many times more expensive than a calcium carbonate filler.
Over the years, numerous attempts have been made to optimise paper compositions precisely for the purpose of improving the porosity and printing properties of the paper. The problem has been, however, that none of these approaches to a solution have been ideal, since they have either had a negative influence on the other properties of the paper (among other things the brightness) or are relatively expensive to use (silicate products).
The use of colloidal PCC as such in paper is known. For example, U.S. Pat. No. 4,892,590 discloses the use of a two-component binder system as a retention agent for paper manufacture, wherein the binder comprises colloidal PCC with a high specific surface area together with a cationic starch. The PCC used has a surface area of 10-200 m2/g, and the weight ratio between PCC and cationic starch is from 2:1 to 1:20.
U.S. Pat. No. 4,460,637 discloses ink-jet paper (coated paper) with 2 different peaks of pore size distribution in the ink-receiving layer or layers. The desired pore size distribution can be achieved, inter alia by means of agglomerates with an average diameter of 1-50 μm in which the individual particles in the agglomerates have a size of at most 0.20 μm, e.g. colloidal particles of at most 0.01 μm; such colloidal particles can be colloidal calcium carbonate.
It is not believed that colloidal PCC has previously been described or used as a filler in paper for the purpose of controlling the porosity and printing properties of the paper.